Abstract

This study investigated the role of a polyphagous insect predator, the Pacific damsel bug (PDB), Nabis kinbergii Reuter [Hemiptera: Nabidae], in the regulation of the Australian crop mirid (ACM), Sidnia kinbergi (Stal)[Hemiptera Miridae], a pest of lucerne seed crops in Canterbury, New Zealand.
Various aspects of POB's predatory behaviour, particularly its searching efficiency, were investigated in the laboratory in detail. The feeding and reproductive capacity of PDB were also examined in the laboratory, while its density, distribution and dispersal were studied in the field.
Despite the temporal synchrony in occurrence and coincidence in spatial distribution of the two species, their relative numbers in the field suggested that PDB was not consuming sufficient ACM to reduce the pest population substantially or to increase its own numbers by a high rate of reproduction. PDB's reproductive capacity was compared when it was reared on each of three prey species. From this it appeared that the food available in the field was not ideal for PDB's maximum reproduction rate, and this probably contributed to low PDB populations. It was also noted that PDB's food requirements for survival are very low. However, laboratory studies of PDB's functional response indicated that it was able to increase its consumption of ACM as the prey density increased. Whether PDB can consume sufficient ACM in the field for survival and reproduction therefore depends on its searching capacity.
It was observed that PDB searches the habitat at random, whether or not prey are present. PDB spent only about 20% of the time searching, and its searching speed was about 3 cm per minute. Vision and olfaction were apparently not involved in prey perception from a distance, and chance appeared to play a major role in contacting prey. Several species found in lucerne, particularly aphids, are acceptable to PDB as food; the presence of alternative prey interferes with predation on ACM.
PDB's slow searching speed, low rate of dispersal in the field (1 m per day), random searching habit and predominantly motionless behaviour suggest that it fails to search the habitat efficiently and thus does not locate a high proportion of the ACM population. Its low food requirements and polyphagous feeding habit indicate that it is not a voracious predator of ACM.
Finally, the overall efficiency of POB as a biological control agent was considered on the basis of the ratios of PDB and ACM observed in the field, as well as POB's searching efficiency as determined in the laboratory. The ACM population observed in this study was much higher than the threshold level (more than 5 ACM per 20 sweeps) suggested by Wightman and Macfarlane (1982), and seed yield loss was 100% in the experimental plot at Kaituna, which was probably due to feeding damage by ACM. No significant effect of PDB in reducing the ACM population was found, and thus PDB is not likely to be an economically important predator.
However, a management programme which may affect the population build-up of ACM and aphids was proposed for use in late November when PDB is absent. Regular monitoring of ACM numbers later in the season was proposed, and if the population exceeds the economic injury level chemical control was recommended.... [Show full abstract]